There are numerous systems in use today for producing cryogenic temperatures. As used herein "cryogenic" temperatures will be defined as temperatures below -150.degree. C. (123.16.degree. K.). This is the value assigned by Russell B. Scott in CRYOGENIC ENGINEERING published by D. Van Nostrand Co., Inc., Princeton, N.J. in 1966 as follows:
"It is rather difficult to assign a definite temperature which will serve as the dividing point between refrigerating and cryogenic engineering, but it will probably conform to present usage to say that cryogenic engineering is concerned with temperatures below -150.degree. C. Another equally acceptable division is to assign to cryogenic engineering the temperature region reached by the liquefaction of gases whose critical temperatures are below terrestrial temperatures". PA1 "A material that contains as an essential ingredient, an organic substance of large molecular weight, is solid in its finished state, and, at some stage in its manufacture, or in its processing into finished particles, can be shaped by flow (definition from ASTM D883-54T)" as defined in the Condensed Chemical Dictionary, Sixth Edition published by Reinhold Publishing Corporation.
Some of the better known cyclicly operating cryogenic systems are the integral and the split Stirling, the Gifford-McMahon and the integral and the split Vuilleumier. Each system operates through the expansion of a compressed fluid and incorporates one or more regenerative heat exchangers which generally comprises a housing with a heat exchanging matrix contained inside. The matrix absorbs heat from a high pressure fluid, usually helium, which flows in a first direction. Heat is stored for a short period and is then transferred back to the fluid, which is at a lower temperature due to expansion, when the fluid is made to flow in the opposite direction, thus completing one cycle. The heat exchange process between the gas and the matrix is essential to the achievement of cryogenic temperatures.
Many applications of cryogenic refrigeration are found today in high technology, highly reliable, long term continuous duty apparatus. Some examples of such apparatus are mazers and parametric amplifiers in communication systems such as satelite or missile tracking systems; superconducting computer circuitry; and high-field-strength superconducting magnets. In such usage efficiency and reliability of the highest order of magnitude is required, and considerations of weight, cost, size and ease of manufacturing are often subordinated to performance and reliability.
As part of the sophisticated technology employed to produce utmost dependability and the highest efficiency, much effort has been devoted to the selection of materials for the regenerative heat exchangers in cryogenic refrigerators. Those which exhibit high volumetric heat capacities at low temperatures are normally preferred. Furthermore, considerable effort has been devoted to the forming or shaping of the heat exchanger matrices after the material has been selected.
Materials often found in cryogenic refrigerating systems include copper, gold, lead, stainless steel, bronze, mercury-lead alloys, nickel, etc. (see U.S. Pat. Nos. 3,397,738, 3,216,484). These metals are intricately fabricated into matrices which can assume various configurations of matrix elements. Some of these are tiny balls or beads, layers of fine wire gauze or mesh, metal wool and stacked perforated disks or plates, to name a few. These metals are not only generally heavy, but they are expensive and the fabrication process necessary to create the matrix is expensive.